In recent years, image information is treated as digital, and at this occasion, for the purpose of transmitting and accumulating information with a high degree of efficiency, apparatuses based on a method such as MPEG (Moving Picture Experts Group) for compression based on orthogonal transformation such as discrete cosine transform and motion compensation by making use of redundancy unique to image information become widely available in not only information distribution such as broadcast station but also information reception at ordinary households.
In particular, MPEG2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) is defined as a general-purpose image coding method, and with a standard covering both of an interlaced scanned image and sequentially scanned image and a standard resolution image and a high-definition image, it is now widely used for wide range of applications for professionals and consumers. When the MPEG2 compression method is used, high compression rate and high image quality can be achieved by allocating, for example, 4 to 8 Mbps as an amount of codes (bit rate) for an interlaced scanned image of a standard resolution having 720 by 480 pixels and 18 to 22 Mbps for an interlaced scanned image of a high resolution having 1920 by 1088 pixels.
MPEG2 is mainly targeted for high image quality coding suitable for broadcasting, but does not support coding method of a less amount of codes (bit rate) than MPEG1. In other words, MPEG2 does not support higher compression rate. As portable terminals become widely prevalent, needs for such coding methods are considered to grow in the future, and in order to respond to such needs, MPEG 4 coding method has been standardized. With regard to image coding method, the specification is admitted as ISO/IEC 14496-2 in international standard on December, 1998.
Further, in recent years, a standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) is standardized for the purpose of image coding for teleconference in the first place. As compared with conventional coding methods such as MPEG2 and MPEG4, H.26L is known to require a higher amount of computation in coding and decoding thereof, but achieve a still higher degree of coding efficiency. In addition, currently, as one of activities of MPEG4, standardization of achieving a still higher degree of efficiency based on H.26L by incorporating functions not supported by H.26L is being done in Joint Model of Enhanced-Compression Video Coding.
With regard to the schedule of standardization, it was made into international standard under the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter referred to as AVC) on March, 2003.
Further, as an expansion thereto, standardization of FRExt (Fidelity Range Extension) including 8 by 8DCT and quantization matrix defined by MPEG2 and coding tool required for business such as RGB, 4:2:2, and 4:4:4 is completed on February, 2005, and therefore, using AVC, this is made into a coding method capable of expressing film noise included in movies in a preferable manner and is beginning to be used in wide range of applications such as Blu-Ray Disc.
However, recently, the needs for coding with a still higher degree of compression rate are growing. For example, it is desired to compress an image of about 4096 by 2048 pixels which is four times the high vision image or distribute high vision image in a limited transmission capacity environment such as the Internet. Therefore, in VCEG under ITU-T as described above, improvement of the coding efficiency is continuously considered.
By the way, making a macro block size of 16 pixels by 16 pixels is not suitable for a large image frame such as UHD (Ultra High Definition; 4000 pixels by 2000 pixels) which is a target of next-generation coding method. Accordingly, as illustrated in FIG. 4, the macro block size is suggested to be made into a size such as 64 by 64 pixels and 32 pixels by 32 pixels (for example, see Non-Patent Document 1).
More specifically, in Non-Patent Document 1, hierarchical structure is employed, and for a pixel block having 16 by 16 pixels or less, a larger block is defined as a super set thereto while maintaining compatibility with a macro block of current AVC.
Non-Patent Document 1 is a suggestion for applying an extended macro block to inter-slice, but aside from this, there is a suggestion to apply an extended macro block to intra-slice (for example, see Non-Patent Document 2).
Further, there is a suggestion for defining an extended macro block using a concept called Coding Unit (for example, see Non-Patent Document 3).
The sizes of the Largest Coding Unit and the Smallest Coding Unit are designated in sequence parameter set in image compression information.